In chargers provided in laser printers, discharge irregularities may occur when foreign matter becomes deposited on a discharge wire of the charger. Such discharge irregularities are one factor leading to a decline in print quality, causing such printing non-uniformities as unintended color shades. Further, if the foreign matter builds up on the discharge wire, the corona discharge may switch to an arc discharge in some cases. This arc discharge can break up the foreign matter on the discharge wire, scattering foreign matter onto a photosensitive drum and resulting in an even greater decline in image quality if the foreign matter becomes deposited on the surface of the photosensitive drum. For this reason, conventional laser printers are commonly provided with a fan or the like on the rear side of the discharge wire. The fan is used to blow air over the discharge wire in order to prevent foreign matter from depositing thereon (see Japanese unexamined patent application publications Nos. HEI-8166697 and HEI-10-198128).
However, since color printers require a plurality of image-forming units, each including a photosensitive drum, a discharge wire, and the like, there are more restrictions on where fans and the like can be positioned, whereas monochromatic printers to which the prior art described above is applied have less restrictions on the positioning of such components. Therefore, the prior art described above cannot be applied in its current form to a color printer.
Further, while blowing air on a discharge wire can reduce the amount of foreign matter deposited on the wire, this technique does not completely prevent the deposition of foreign matter.